It is known to provide a backflow preventer or checkvalve at the downstream end of a plastifier screw or worm in an injection molding machine to prevent backflow of the liquified or molten synthetic resin material as the injection molding pressure is built up ahead of the worm.
In general the injection molding machine can comprise the plastifier worm, and means for rotating this worm to homogenize, plastify and liquefy the thermoplastic synthetic resin material which is introduced into the cylinder in which the worm is rotated.
During the plastifying process, the material is advanced to a location ahead of the worm, usually past a backflow preventer or checkvalve, whereupon other means can be activated to axially displace the worm and, utilizing it as a sort of piston, to drive the material ahead of the worm into a mold (see U.S. Pat. No. 3,319,299).
As the injection molding pressure is built up ahead of the work, the checkvalve closes.
While various checkvalve or backflow preventer systems have been developed for this purpose, the present invention is only concerned with those which comprise a forwardly tapering or conical body secured to the downstream end of the worm, and a blocking ring disposed between this body and the worm and free to move axially between blocking and unblocking positions.
A pressure ring may be provided against the end of the worm to form a seat which is engaged by the blocking ring in the closed position of the valve.
In the aforementioned U.S. patent, the pressure ring is threaded onto a stem of the torpedo-shaped body and rests against a shoulder in the direction of flow of the synthetic resin material while the body is formed with channels effective to pass the synthetic resin material in the open position of the valve.
The forces generated by the pressure of the blocking ring against the pressure ring are all taken up directly by the stem of the torpedo body so that this stem is susceptible to damage. Of course damage to the stem can be avoided by increasing the diameter thereof, but any such increase in diameter will invariably result in a decrease in the annular space available to form the flow passage for the synthetic resin material.
An attempt is made to avoid this problem by applying the pressure ring of German open application No. 2,635,144 to the end of the worm, the shank of the conical body having a shoulder which bears upon the pressure ring. The shoulder helps to distribute force to the pressure ring and forces traversed from the blocking ring to the pressure ring and then to the end of the worm in the closed position of the valve.
This assembly, of course, improves the ability of a stem to withstand the forces generated, but it also represents a problem since here the pressure ring constitutes most of the force transmitting area, especially if it is used to establish the depth to which the stem is threaded into the end of the worm.
Reference may also be had to the backflow preventers described in German printed application No. 20 06 389 and in German open applications Nos. 30 09 399 and 28 36 165.
These checkvalves have either blocking rings movable relative to the worm or a head which is movable relative to the worm.
In all of the prior art systems thus far described, and even that of German open application No. 25 15 530 which has a movable head, it is understood that the checkvalves should be opened and closed rapidly and thus, that the surfaces exposed to the synthetic resin pressure which causes the opening and closing should be as large as possible.
Furthermore, it is advantageous to have large sealing surfaces and to be able to adjust the flow passage of the valve for different rheological properties of the synthetic resin material in the open position thereof.
The prior art valves have had various drawbacks, some of which have been dealt with previously but others of which have not yet been fully attached.
Obviously when only the stem of the conical body of the backflow preventer or valve cap of transferring force to the worm through a screw or the transfer of force is effective only through threads of a screw or by a pressure ring, the cross section of the worm over which large forces are transmitted must be comparatively small and further cannot be readily enlarged without reducing the surface areas exposed to the synthetic resin material and providing the rapid action which is required. In addition, the prior art devices are not readily, conveniently and accurately adjustable for the purpose of varying flow cross section in accordance with rheological properties.